It is known that the knowledge of the speed of an aircraft relative to the air is paramount for the piloting of this aircraft. So, the crew monitors this speed cue (or value) very closely so as to keep the aircraft in a flight domain defined by minimum and maximum speeds, for which this aircraft was designed. For a given flight point, below the minimum speed, the aircraft is at risk of stalling, and above the maximum speed, it is subject to strong structural loads, for which it was not designed. It is therefore important that the crew should have accurate and reliable knowledge of the speed cue.
Moreover, it is also known that aircraft, and in particular airplanes such as transport airplanes, for example, are equipped with various probes designed to measure a certain number of parameters. These parameters are thereafter transmitted to onboard instruments. Thus, certain probes, located in proximity to the engine of the aircraft, provide information cues making it possible to follow the behavior of said engine (gas ejection temperature, revs, etc.) or to deliver information cues relating to the environment in which the aircraft is maneuvering (static pressure, total dynamic pressure, temperature, etc.). Other probes which are installed on the fuselage deliver only pressure or temperature information cues representative of the environment in which the aircraft is maneuvering. These probes are redundant and sited at different places on the aircraft, so as to avoid common faults (fault with several probes having a single cause). They use, for example, anemometers.
There are in particular probes, for example so-called “pitot” probes, which are intended for measuring dynamic pressure and probes, for example so-called “pepperpot” probes, which are intended for measuring static pressure.
The pressure values measured by the various probes, are in particular used by appropriate computers to compute speed cues of the aircraft.
These speed cues, namely, in general, three different speed cues per aircraft, are thereafter used by a flight control computer of the aircraft to compute flight control orders for maneuvering the aircraft. This flight control computer verifies, before using said speed cues, the consistency of these cues. If it deems that a speed cue is erroneous or spurious (for example, when the difference between this cue and the others is greater than a certain threshold), it discards this speed cue for the entire duration of the flight and now monitors only the latter two. In the case where the difference between these latter two speed cues becomes greater than a certain predefined threshold, the flight control computer definitively discards, for the remainder of the flight, these two cues, since it does not know which one has become spurious. In this case, said flight control computer switches to a degraded mode and alerts the crew that the speed cue available is no longer reliable. The aircraft is now governed only by so-called “degraded” control laws which no longer consider the speed cue and which are therefore less accurate and powerful.
However, there are faults that may go totally unnoticed at the flight control computer level, but these faults may have repercussions on the piloting of the aircraft, which may turn out to be very penalizing.
Thus, during maintenance operations, it is frequently the case that the “pitot” probes are made secure by protecting them with a hood, or even that the “pepperpot” probes are disconnected. It may happen that, through an omission of the maintenance personnel, the aircraft takes off although the probes have not been reconnected or their hoods have not be removed. They are therefore no longer operational and values which are delivered and provided to the computers are no longer representative of the environment in which the aircraft is maneuvering.
It is also conceivable that, although not being operational, two “pepperpot” probes may deliver mutually consistent speed values or cues which are completely erroneous, however. The mutual consistency of the speed cues then induces the flight control computer to discard the value delivered by the sole operational probe (the third “pepperpot” probe which in this situation produces the sole value representative of the environment in which the aircraft is maneuvering) in favor of the values delivered by these two “pepperpot” probes (delivering erroneous values). In this case, the flight control computer computes control orders with erroneous values and the crew trusts a speed cue that is not reliable.
Such a situation is very dangerous. Specifically, if the speed cue provides a speed which is too slow (respectively too fast), the crew will be led to accelerate (respectively to decelerate), thereby running the risk of making the aircraft fly in a domain of unauthorized speeds. It may also happen that the speed is such that the control laws do not cover this case, and this may lead to the loss of control of the aircraft.
Moreover, a fault with a static or total pressure measurement probe may also be produced by aerological phenomena which are sometimes encountered in flight (icing of the probes of the fuselage) or by the obscuring of said probes by dust, insects or other foreign bodies.